The mainstay of electronic imaging, since its beginning, has been the cathode ray tube (CRT) or kinescope. Although CRT technology has progressed over the years, several major drawbacks remain. Picture size is still limited, making group viewing difficult. CRT picture tubes larger than about 30" (measured diagonally) become impractical because of size, weight, expense and danger of implosion because of the high vacuum used. To achieve high brightness, they use dangerously high voltages and may produce health hazards from x-rays and electromagnetic fields.
Image quality of CRT-based video displays may be degraded by color distortion, image shape distortions, color impurity from the influence of the earth's magnetic field, and color misconvergence. In addition, CRT displays are subject, particularly when viewed at close range, to visual artifacts such as scanning lines and discrete phosphor dots or stripes, which are inherent in such TV displays. These visual artifacts provide a poorer image quality than images in movie theaters.
Research has been continuing on for many years to develop other types of light emissive displays which would overcome some of these drawbacks. Plasma, electroluminescent (EL) and cold cathode phosphor displays are among the most promising candidates, although they have not proved themselves to be practical. Furthermore, it is highly questionable whether these other emissive displays, if and when successful, would provide any advances over current CRT brightness or size in practical applications.
"Pocket Tvs" with a 2"-3" picture are constructed today using liquid crystal displays which are addressed via electronic multiplexing or active matrix addressing. Creating a large picture for direct viewing however poses many problems which have heretofore not been overcome. Simple multiplexing cannot produce a satisfactory image because of cross-talk. An active matrix relieves the cross-talk problems, but has so many more production steps and so many switching and storage elements that must be deposited over a large surface area that production of large, defect-free active matrix displays for direct viewing has not been possible and may never be economically feasible for very large displays.
Demand for large video imaging systems and for thin profile or "flat screen" imaging systems, both large and small, has increased significantly in recent years and is expected to increase dramatically with the advent of high definition television broadcasts. "Projection televisions" have been developed and commercialized in recent years. Unfortunately, such projection display devices have exacerbated many of the problems associated with earlier video display systems and have created new problems. Projection televisions are more expensive than standard direct-view televisions and are more cumbersome, heavier, and larger so that portability is impractical. Two types of projection television systems have become popular: one using three CRTs with projection lenses and the other using an oil film scanned by an electron beam.
The CRT-based projection system remains relatively dim, requiring a dimly-lit viewing environment and a costly special screen which provides a very limited viewing angle. The three CRTs produce images in the primary colors, blue, green, and red and are driven with higher anode voltage than conventional systems to obtain as much brightness out of them as possible. The higher anode voltage lowers tube life and increases the radiation hazards and other problems associated with high voltage. The three tubes also increase the danger of tube implosion. The standard oil-based system, referred to as an Eidophor, has three "scanned oil elements" which have a relatively short life and use external light sources. In either system, all three color images utilizing three sets of optics must be precisely converged onto the viewing screen, in addition to requiring adjustments of hue, saturation, vertical and horizontal size and linearity, and minimization of pincushion and barrel distortion. Proper alignment in either system is therefore beyond the abilities of the average person. Proper convergence is not easily achieved and often requires up to a half hour of additional set-up time because of the curvature of the lenses and variations in the performance of the circuits in either system. If the projector or screen is moved, the convergence procedure must be repeated.
Experimentation has also been performed on laser systems which scan out an image on a viewing screen in the same way an electron beam scans the image onto the face of a CRT. The laser systems developed thus far are much too large to be portable, very complex to use and maintain, extremely expensive, potentially dangerous and have proven too dim for large images.
Many attempts have been made to solve the above-mentioned problems, resulting in experimentation on several novel "light valve" based systems. This type of system uses an external light source which can theoretically be as bright as desired, with a "light valve" to modulate the light carrying the picture information. The research and experimentation to develop a workable light valve system has been primarily directed to using different optical, electronic, physical and other effects and finding or producing various materials to accomplish the desired results. The various light-valve system attempts have mainly utilized crystals (such as quartz, Potassium Di-Hydrogen Phosphate, Lithium Niobate, Barium Strontium Niobate, Yttrium, Aluminum, Garnet and Chromium Oxide), liquids (such as Nitro Benzene) or liquid crystals (of the smectic or nematic type or a suspension of particles such as iodoquinine sulphate in a liquid carrier) or other similar materials to take advantage of one or more optical effects including electro-optical effects, such as creating a rotated plane of polarization or altering the index of refraction of the material due to an applied electric field, magneto-optical effects using an applied magnetic field, electro-striction effects, piezo-optical effects, electrostatic particle orientation, photoconductivity, acousto-optical effects, photochromic effects and laser-scan-induced secondary electron emission. Except for liquid crystal light valves, such light valves proved impossible to manufacture economically and with a sufficiently large aperture and have often been toxic, dangerous, and inconsistent in production quality.
In all light valves, different areas must be supplied different information or "addressed," so that a different amount of light emerges through each area, adding up to a complete picture across the total beam of light. Techniques for addressing different picture elements (or "pixels") of a light valve have included methods for deflecting a laser or electron beam to that area or the use of a tiny criss-cross of electrically conductive paths, i.e., a matrix, deposited on or adjacent to the material to be addressed in order to activate that area of the matrix. In scanning beam systems, problems have included outgassing and erosion of material. The electrical matrix system has proved difficult to engineer, requiring deposition with extremely high precision of a transparent material having good conductivity characteristics. Further, such matrices must be driven by extremely fast switching circuits, which are impractical at the high voltages required to activate a given area of most materials.
The most frequently used system for addressing small areas is often referred to as electronic multiplexing. Electronic multiplexing works well with only low voltage-requiring materials such as liquid crystals. With this method, all pixel addresses are x and y coordinates on the conductive grid. To activate a given pixel area a specific amount, different voltages must be applied to the x and y conductors so that, where they meet, they together exceed a threshold voltage and modulate the area. A major drawback to such multiplexing is cross-talk, where surrounding areas are affected by the electric field, causing false data to influence surrounding pixels, reducing contrast and resolution, as well as color saturation and accuracy. The cross-talk problem increases when resolution increases because liquid crystal materials respond fairly linearly to applied voltage. Since all pixels are interconnected within the same system, all pixels are given partial voltage and are, thus, partially activated when any one pixel is addressed. Non-linear materials can be added to the liquid crystal mix, but this still doesn't allow for more than about 160 lines of resolution before cross-talk significantly degrades the image.
An "active matrix" light valve in which all pixels from the matrix are selectively disconnected except for those pixels which are addressed at any given time eliminates the cross-talk problem, regardless of the number of pixels or lines in a display. Recently, active matrix displays have been made utilizing transistors, diodes, or an ionizing gas as the switching element to disconnect the pixels.
Since liquid crystal light valves have very little persistence and one pixel or line of pixels is activated at a time, substantially less light is projected to the screen to be ultimately viewed since all pixels are "off" most of the time. This characteristic wastes light, produces a dimmer image with poorer contrast and generates more heat because of the brighter source necessary to compensate for the dim image. High refresh rates are impractical because they would require faster switching times and faster responding material.
Active matrix displays, however, also utilize a storage element, such as a capacitor, connected to each pixel, which allows each pixel to retain the proper charge, and thus, the proper transmissivity after the pixel has been addressed and disconnected from the system. Thus, each pixel remains "on" the correct amount all the time. This increases light throughput and eliminates flicker.
If high-wattage light sources are used in order to achieve very bright displays, heat sensitivity can cause a decrease in contrast and color fidelity. Absorption of high intensity light by color filters and polarizers (if used), even if little or no infrared light is present, results in heating of these elements which can also degrade image quality and may even damage the light valve. Use of fan cooling causes objectionable noise, especially in quiet environments when source volume is kept low.
Another inherent problem of light valve projection systems relates to the fact that each pixel of the frame is surrounded by an opaque border that contains addressing circuitry or physical structure. This results in visibly discrete pixels and contributes an objectionable "graininess" to the image that become progressively more annoying when viewed at close distance or on large screens. The problem is amplified if a single full-color light valve is used in which the individual red, green, and blue color elements of each pixel are not converged or blended and are visible to the viewer.
Consequently, projection by means of a small light valve provides the most practical and economical way to produce a large, bright image. Unfortunately, such light valve projectors have, up to the present, exhibited several shortcomings which fall generally into at least four broad categories, namely:
1) light valve restrictions; PA1 2) light source limitations; PA1 3) optical system inefficiencies; and PA1 4) screen performance weaknesses.
These problems must be addressed to allow for the successful production of acceptable quality, practical display systems, capable of large projection imagery and display of small or large images from a device with a "thin profile."
To address these and other problems associated with prior art video display systems, it is an object of the present invention to provide an adjustable size video image which can be very large, yet possesses high quality and sufficient brightness to be visible from wide viewing angles without distortions, in a normally lit room as well as in environments with high ambient light.
Furthermore, an object of the invention is to create a video display system which utilizes a light valve such as a specially constructed LCD light valve, an independent light source with a long life, high brightness, average luminance, and color temperature, and novel optics, providing for high light efficiency for front or rear projection and which operates without excess heat or fan noise.
Another object of the invention is to produce such a system with high resolution and contrast (eliminating the appearance of stripes, pixels, or lines), with highly accurate color rendition (equal to or better than that of a CRT).
An additional object of the invention is to produce a display that reduces eye strain by the elimination of flicker and glare and by the broadening of color peaks.
A further object of the invention is to produce a small, lightweight, portable system, having a long maintenance-free operating life, which is operable in conjunction with or without a special screen and can be mass-produced relatively inexpensively.
Yet another object of the invention is to produce a system which requires no convergence or other difficult adjustments prior to viewing.
Still another object of the present invention is to produce a system with greatly reduced radiation and hazard of tube implosion and operates with relatively low voltage.
An additional object of the invention is to produce a system which does not require a special screen, can be easily projected onto a wall or ceiling, and can be viewed comfortably at relatively wide angles.
A further objective of the invention is to produce such a system capable of three-dimensional projection.
Additional objects of the invention include the creation of a system which will overcome drawbacks associated with CRTs in terms of weight, bulk, high voltage, radiation, implosion hazard and convergence difficulty in 3-CRT projection systems.
Further objects will include increasing image contrast, color reproducibility, resolution and yield while reducing color pixel visibility, flicker, heat sensitivity, image artifacts, system cooling noise and bleed-through of non-image bearing light, while decreasing the cost and complexity of light valve systems.
Additional objects of the invention involve creating a system to overcome and improve upon light source limitations by increasing brightness efficiency, average luminance and color temperature, while lengthening bulb life and reducing the weight and bulk of the power supply.
Yet additional objects of the invention involve creating a system with improved light collection, decreased light losses due to color selection and polarization, decreased light valve aperture ratio losses and other non-image light waste.
Further objects of the invention involve creating a system which involves improving performance by use of particular screen materials with reduced light absorption, while reducing lenticular-lens-pattern image degradation, off-axis projection distortion and off-axis brightness fall-off, while reducing the effect of glare and ambient light to image visibility.
Moreover, it is an object of the invention to create a system which minimizes and virtually eliminates the wasted space of projection distance and enables three-dimensional projection.
Other objects will become evident from the disclosure.